The idea of function-retaining artificial replacements for intervertebral discs is younger than replacements of artificial joints of extremities, but nonetheless more than 50 years old [Büttner-Janz, Hochschuler, McAfee (Eds.): The Artificial Disc. Springer Verlag, Berlin, Heidelberg, New York 2003]. It is a response to biomechanical considerations, unsatisfactory results of fusion surgeries of the cervical and lumbar spine, disorders adjacent to fused motion spinal segments, and the development of new materials with better sliding properties and greater longevity.
By means of function-retaining disc implants it is possible to avoid fusion surgery, i.e. to maintain, to restore or to improve the mobility within the intervertebral disc space. In an in-vitro experiment it is possible to achieve a normalization of the biomechanical properties of the spinal motion segment to a large extent through the implantation of an artificial intervertebral disc
Presently, many intervertebral function retaining disc implants are clinically used. Total disc replacement on the lumbar spine started with the CHARITE Artificial Disc, later followed by the PRODISC, the MAVERICK, the FLEXICORE, the MOBIDISC, the KINEFLEX; the ACTIV L, the XL-TDR, the DYNARDI, the PHYSIO-L, the INMOTION, the M6L, the FREEDOM and further disc prostheses. On the cervical spine many function retaining disc implants are known as well, as for example the BRYAN, the PRESTIGE, the PRODISC-C, the KINEFLEX C, the MOBIC, the ACTIV C, the DISCOCERV, the DISCOVER, the PCM, the CERVICORE, the M6C, the GALILEO, the GRANVIA, the NUNEC, and the BAGUERA C.
There are different classifications of cervical and lumbar total disc prostheses, according to the number of articulation partners, and according to biomechanical considerations and direct function-related conditions within the cervical as well as the lumbar intervertebral space. At present, prostheses with two, one or no articulation surface(s) are used. Depending on the number of functional components and the material, the prostheses have biomechanically a fixed or mobile centre of rotation. Whereas prostheses with two sliding surfaces having a mobile centre of rotation are more physiologically designed, prostheses with only two functional related partners and one sliding surface are able to better stabilize the spine in multi-segmental implantations.
The artificial discs can be implanted via a ventral, ventro-lateral, lateral or dorsal approach. Depending on the approach the artificial discs are constructed with different shape, size and means for instruments. Different sizes of disc prostheses base on the size of the prosthetic plates, different heights on the height of the prosthetic components, and different angles of lordosis on the angles of prosthetic components. The trapezium natural disc shape is primarily responsible for the lordosis of the lumbar and cervical spine, further the vertebral bodies contribute to a minor extent to the lordosis. During prosthetic replacement of an intervertebral discs the lordosis should be maintained or reconstructed. A hyperlordotic angle of the operated spinal segment, because in the longer run a painful facet joint degeneration can be expected, is to avoid. A hyperlordotic disc space is a pre-condition for reduced segmental range of motion as well. In that case the target prevention of the adjacent motion segment against disc degeneration can not be fulfilled.
The common material for sliding disc implants is metal in combination with polyethylene or metal-to-metal. In the meantime partly new materials for total discs are used including for coating of the disc implants to achieve the opportunity for MRI diagnostics as well as better sliding properties of the implants, to avoid any revision surgery.
A healthy intervertebral disc allows in its interaction with other elements of a functional spinal unit limited motion at different ranges of motion in extension and flexion as well as in lateral bending to the right and left and in axial rotation. Motion to the front and back is combined with rotational motion, and side motion is combined with other motion directions; it is a matter of so called “coupled motion”. The motion amplitudes of healthy intervertebral discs are different, with respect to extension (bending back) and flexion (bending forward) as well as to the lateral bending to the right and left and axial rotational motion. Although of common basic characteristics, there are differences between the motion amplitudes of the cervical and lumbar spine as well.
All prostheses for total disc replacements which are currently clinically used do not cover completely the natural function of a cervical or lumbar motion segment, including the natural range of motion. In the long-term experience facet joint degeneration and facet joint disease at the same level of disc implantation and/or in the neighbourhood may occur as a result of prosthetic disc hyper-mobility and connected dysfunction of the facet joints. Abrasion of the facet joints (arthritis, spondylarthritis) may occur, with a formation of osteophytes. As a result the irritation of neural structures is possible as well as directly caused pain in the facet joints.
There is so far no evidence that shock absorption is needed within the spine to save tissue or specific anatomical structures; the facet joints are usually not horizontally loaded. The main function of shock absorption seems to enable a motion within the disc space because the natural disc does not have typical joint partners like ball and socket. By having the shock absorption of the disc the intervertebral angle can be changed without reducing significantly the disc height at same time.
Some disc prostheses include shock absorption to its function, but again without to simulate the segmental physiological range of motion. Postoperatively kyphotic intervertebral disc spaces with dangerous potential for facet joint hyper-mobility and facet joint degeneration and disease can be observed. So the original aim of a function retaining disc replacement to achieve a painless or pain free stabilization of the spinal motion segment by implantation of the disc prosthesis is not yet fulfilled in the long run.
The longest experience exists with the Charite prosthesis, which is the subject matter of DE 35 29 761 C2 and U.S. Pat. No. 5,401,269 specifications. This prosthesis was developed in 1982 by Dr. Schellnack and Dr. Büttner-Janz at the Charite University Hospital in Berlin and was later on named SB Charité prosthesis. In 1984 the first surgery took place. The intervertebral disc prosthesis was further developed into model III and has been implanted worldwide (DE 35 29 761 C2, U.S. Pat. No. 5,401,269) since 1987; it is replaced by the INMOTION, with same functional principle of a three-part metal to polyethylene prosthesis, with two identical spherical articulation surfaces and a mobile centre of rotation.
Due to a simultaneous translation movement of adjacent vertebrae, the centre of rotation changes its position constantly in case of inconstant centre of rotation. The prosthesis according to DE 35 29 761 C2 shows a construction which differs relative to other available types of prostheses which are build like a ball and socket joint and as a result move around a defined localized centre of rotation. By virtue of the three-part assembly of the prosthesis according to DE 35 29 761 C2, with two metallic endplates and the interpositioned freely mobile polyethylene sliding core, the course of motion of a healthy intervertebral disc of the human spine is mimicked as far as possible, however without the exact motion amplitudes in the specific motion directions.
There is a need for an intervertebral disc prosthesis for the cervical and lumbar spine that enables physiological motion of high quality and physiological quantity. The three-dimensional range of motion within the intervertebral space should mimic the coupled motion of a natural disc, including the physiological translation in the sagittal and frontal view. In case of special anatomical and/or biomechanical local conditions or in case of revision surgery, a disc prosthesis is sometimes needed which does not allow motion to all directions postoperatively. A disc prosthesis for implantation via a lateral approach is needed as well.
In case of any problem after total disc replacement, finally a fusion surgery is often carried out, sometimes including removal of the disc prosthesis. Especially in disc prostheses with big anchoring means for fixation on the vertebral body the removal of the implanted and ingrown disc prosthesis leads to the need of bone removal at same time to make the explantation of the disc prosthesis possible. Depending on that issue, a combined usable total disc prosthesis is needed for preservation of motion and if needed for fusion surgery according the surgeon's decision pre- or intraoperatively, to exchange a prosthetic component without removal of the fixed prosthetic plates for having no intervertebral motion postoperatively.
There are only a very few prostheses with luxation-protected components known, as for example the FLEXICORE. Having a prosthesis which also includes physiological translation and which avoids any luxation of components at e.g. a whiplash after accidents via its desing or construction, would be a big new advantage of total disc replacement, allowing more sportive activities or even surgeries on pilots.